1. Field of the Invention
The present invention relates to an image reading apparatus for use in a digital copy machine or an image scanner.
2. Description of Related Art
The development of light emitting diode (hereinafter called LED) has been actively conducted in recent years. The brightness of the LED element has rapidly grown. The LED generally has advantages such as longer operating, high efficiency, high anti-G performance, and emission of a single color, so that its application to various fields of illumination is expected. As an application of the LED, the LED is used in image reading apparatuses and original document illumination apparatuses such as digital copiers or image scanners.
As an application of the LED for use in the image reading apparatus, various methods have been proposed. For example, in JP 2005-241681A, a large number of the LEDs is arranged parallel to a main-scanning direction of a surface of an original document, the light emitted from the LEDs is diffused by not providing an optical effect in the main-scanning direction, and the light emitted from the LEDs is concentrated by the concentration effect of a reflective face in a sub-scanning direction, so as to increase the illumination intensity of the reading target area on the surface of the original document.
Moreover, in JP2693098B, light emitted from LEDs is led to the main-scanning direction by repeating the total reflection in a light guide to be diffused and reflected in a diffusion and reflection area, and the light deviated from a total reflection condition is emitted to the outside of the light guide. By way of this, the uniformity of the illumination distribution in the main-scanning diction is increased.
Furthermore, in JP2004-361425A, a large number of LEDs is arranged parallel to a main-scanning direction of an original document, the light emitted from the LEDs is diffused by not providing an optical effect in the main-scanning direction, and the light emitted from the LEDs is concentrated by the concentration effect of a lens in a sub-scanning direction, so as to increase the illumination in the reading target area on the surface of the original document.
However, if an open book or the like is placed on a contact glass (an original document platen), a portion between pages is inevitably raised because of the structure of the book or the like. For this reason, the raised portion between pages forms an angle relative to the contact glass. The entire raised portion having an angle can not be illuminated by the illumination from one direction because a shadow is formed. Accordingly, there is a problem in that a black shadow is read when reading the original document.
Conventionally, as described in JP2004-361425A, with a light source using a cold-cathode tube, illumination from two directions in a first area and a second area is achieved by placing reflection plates opposed to each other. In this case, from a passage of light reflected from the original document and used for imaging, the light source side is defined as a first area and the side opposite to the light source is defined as a second area.
However, in the constitution described in JP2005-241681A, although the light use efficiency can be improved by utilizing the reflective face, the surface of the original document is illuminated only from a first area side. Since the light emitted from the LEDs is concentrated by the reflective surface, even if opposed reflective plates are disposed in a second area, almost no light passes to the opposed reflective plates. For this reason, the illumination from the two directions is practically impossible. If the illumination from the two directions is achieved by lowering the concentration degree, the light use efficiency is severely decreased.
Moreover, in the constitution described in JP2693098B, the light-emitting quantity of the optical illumination system is completely decreased relative to the method of arranging the LEDs because the number of the LEDs which can be provided in one light guide is limited. Therefore, in the constitution described in JP2693098B, it is desirable to provide the optical illumination system in each of the first area and the second area, so as to perform the desired illumination from the two directions, but this generates high costs.
Furthermore, in the constitution described in JP2004-361425A, the size of the entire optical illumination system in a first mirror direction from the surface of the original document surface tends to increase.